This invention relates generally to melt-extrusion lamination techniques for producing extrusion-laminated films. More specifically, the invention relates to the production of laminated films for lateral stretching or tentering by grasping the lateral edges thereof by means of clamping tools, that is, clips, and relates to a method and apparatus for producing laminated films which can be easily clamped by clips.
The method of melt extruding on one surface or both surfaces of a previously formed resin film (substrate film) a resin of the same kind or a different kind (i.e., different in properties such as chemical properties or melt index, etc.) and subjecting the films suitably to pressure and cooling thereby to produce a laminated film comprising the two films bonded together is known.
On one hand, the procedure of stretching a single-layer film or a laminated film of a resin at a temperature above the second-order transition temperature and below the melting point or the temperature at which the resin becomes fluid is also known.
In the case where a film of finite width and infinite length is continuously stretched in its longitudinal direction (i.e., the machine direction), a tension is applied to the film passing through a heating zone at points before and after this heating zone, but in the case where the film is stretched in the width direction, that is, in the transverse or lateral direction the film is passed through the heating zone as the lateral edges of the film are clamped by suitable tools and a tension force is applied thereto in the transverse direction.
While various kinds of clamping tools for gripping the lateral edges of a film to be stretched laterally are conceivable, one representative example thereof is a clip device as shown in FIG. 1, a plurality of which is used along each lateral edge of the film to stretch the film uniformly in the transverse direction. This clip device comprises, essentially, a C-shaped clip frame structure having a fixed jaw or anvil 1A and an upper head 1B disposed above and spaced apart from the anvil 1A, a horizontal pin 2 passing through the upper head 1B of the clip, and a lever 3 pivotally supported by the pin 2 and having a handle arm extending upward from the pin 2 and a clamp jaw arm extending slightly obliquely downward and terminating at a clamp jaw 3A.
The lateral edge 4 of a film entering the clip in the direction of the arrow X is clamped by and between the anvil 1A and the clamp jaw 3A. When the entire clip device 1 is moved in the arrow direction Y, the clamp jaw 3A at the lower end of the lever 3 is forced by a semi-toggle action against the upper surface of the anvil 1A, whereby the film lateral edge 4 is gripped strongly and a tension in the transverse direction is imparted thereto.
In the production of a film by melt extrusion of a resin, a so-called "neck-in" phenomenon occurs and gives rise to a reduction in the film width and an increase due thereto in the thickness of the film along its lateral edges or a so-called beaded edge effect.
These phenomena occur also in the case of melt-extrusion lamination. In the case where the base or substrate film has been produced by melt extrusion, this substrate film also has beaded edges as long as these lateral edges are not trimmed. Consequently, the lateral edge parts of melt-extrusion laminated films to be clamped by clips as described above become considerably thick.
On one hand, since the distance D and the length of lower clamp jaw arm of the lever 3 below the pin 2 are fixed in a clip of the above description, there are limits to the thickness of films which can be clamped by a given clip. More specifically, when the film thickness t becomes excessively large the angle .theta. becomes excessively large, whereby the gripping force between the anvil 1A and the jaw 3A does not become sufficiently large. On the other hand, a film which is thinner than the gap distance between the anvil 1A and the jaw 3A when the angle .theta. is zero cannot be gripped by this clip. While the jaw length from the pin 2 to the jaw 3A may be greater than the distance D, the tendency of the angle .theta. to become excessively large in this case is greater than that in the case where this jaw length is less the distance D.
In order to avoid a drop in the gripping performance of the clip occurring in this manner, the bead-edge effect of the film part gripped by the clip is suppressed as much as possible, and a clip having a great distance between the anvil 1A and the jaw 3A is selected to permit the clip to grip a thick film. Such measures have heretofore been known.
However, these measures are not fully satisfactory. That is, suppression of the bead-edge effect imparts some kind of change in the melt-extrusion lamination conditions, and, at the same time, a change appears also in the lateral stretching behavior. The latter measure entails a drop in the productive efficiency due to diversification of clips. It is conceivable to provide a bead on each lateral edge of the film for the purpose of providing a part which can be more easily caught by the jaw 3A, but when such a thick bead exists at the film lateral edges, the film cannot be made to contact uniformly the surface of a cooling roll in the case where the film is to be cooled. As a result, the cooling becomes uneven, whereby the stretching also becomes non-uniform, and tearing due to stretching and other difficulties is caused.
Moreover, even if these problems were to be solved, the film gripping action of this type of clip would not necessarily be positive, and difficulties due to insufficient gripping would tend to occur in a lateral stretching or tentering operation.
Accordingly, there is a great need for improvements in the art, particularly in lateral stretching with the use of clips of this type, such as expansion of the range film thickness (i.e., the lateral edge thickness) which can be accommodated without specially varying the conditions of melt-extrusion lamination and, moreover, fundamental improvement of the gripping capability of the clips.